Ultrasound transceiver and control of a thermal damage process

ABSTRACT

An ultrasonic transceiver apparatus for intracorporeal use, comprises an undamped ultrasonic transceiver for placing in a confined intracorporeal space, the transceiver vibrating at an instantaneous frequency and being excited to cause vibration at said instantaneous frequency to produce an ultrasonic ablation beam for ablating surrounding tissues and being further excited to cause vibration by primary echo signals returning from surrounding tissues—from separate excitations during quiet periods between the ablation—to monitor ablation progress. A signal processor isolates the primary echo signals from ringing, secondary echoes and extraneous noise also received from the transceiver and uses presence or absence of the characteristic frequency, or of a body characteristic frequency such as pulse or breathing, as an isolation criterion.

The present application claims priority from U.S. Provisional PatentApplication No. 61/393,947 filed Oct. 18, 2010, U.S. patent applicationSer. No. 13/049,022 filed Mar. 16, 2011 and U.S. Provisional PatentApplication No. 61/453,239 filed Mar. 16, 2011.

The present application is related to co-filed, co-pending andco-assigned PCT patent applications entitled:

“Therapeutics Reservoir”, (attorney docket no. 52341) relating torelates to a method of drug delivery and, more particularly to a methodfor trapping drugs to form a drug reservoir in tissue.

“Ultrasound Emission element” (attorney docket no. 52344), showing, forexample, an apparatus for generating relatively high efficiencyultrasound;

“an ultrasound transceiver and uses thereof” (attorney docket no.52345), showing for example, a method for feedback and control of theultrasonic transducer;

“an ultrasound transceiver and cooling thereof” (attorney docket no.52346), showing for example, a method for blood flow cooling of theultrasonic transducer;

“tissue treatment” (attorney docket no. 52347) by Ariel Sverdlik, ErisSzwarcfiter and Or Shabtay, showing for example, a method of selectivetargeting and treating tissues using ultrasound; and “separation devicefor ultrasound element” (attorney docket no. 52348) by Ariel Sverdlikand Or Shabtay, showing for example, a device to prevent the transducerfrom touching the blood vessel wall.

The disclosures of each of the above are incorporated herein byreference.

FIELD AND BACKGROUND OF THE INVENTION

The present invention, in some embodiments thereof, relates to anultrasound transceiver and to control of an ablation or thermal damageprocess to a tissue and, more particularly, but not exclusively, to useof the transceiver in simultaneous monitoring and ablation of nearbytissue by monitoring the distance to the tissue walls, and use of changein distance between tissue walls as an indicator of progress of thethermal damage.

For the purposes of ablation of tissue it is desirable to have anultrasound transmitter which is undamped, because any damping reducedefficiency.

For the purpose of detection of effects, a damped receiver is desirable,since without damping, ringing occurs, making signals hard to read.

In the case where the same device is used both for ablation anddetection, the situation is particularly bad, since the transmittedsignals cause considerable ringing in the undamped device.

The problem is to detect the wall of an artery, or any other tissue,using an ultrasonic transceiver which is also performing the ablationand which thus must be undamped because damping will reduce efficiency.Low efficiency leads to increased heating, which can be harmful inconfined spaces such as blood vessels.

Since the detector is undamped there is considerable noise due toringing. The ringing is present in the sensor and is also transmitted tothe tissues causing reflections of its own, all this interfering withthe signal it is intended to detect.

As a consequence, the signal from the artery wall is drowned out bynoise due to ringing and the primary echo is hard to discern.Nevertheless, detecting the echo is highly desirable in order to monitorthe ablation treatment.

SUMMARY OF THE INVENTION

An embodiment of the present invention may analyze a sequence of samplesignals. The primary echo has a fixed relationship to the excitationsignal, whereas the ringing signals do not. Typically the primary echoshares a main frequency component with the excitation signal althoughnot the phase and not the amplitude. The relationship is used todistinguish the primary echo from ringing and from secondary echoes.

According to an aspect of some embodiments of the present inventionthere is provided an ultrasonic transceiver apparatus for intracorporealuse, the apparatus comprising:

an undamped ultrasonic transceiver for placing in a confinedintracorporeal space, the transceiver having an instantaneous excitationfrequency and for receiving excitation at the excitation frequency toproduce an ultrasonic ablation beam for ablating surrounding tissues andbeing further for receiving excitation by primary echo signals returningfrom surrounding tissues;

a signal processor connected to the transceiver configured to isolatethe primary echo signals from ringing, secondary echoes and extraneousnoise also received from the transceiver, the signal processor usingpresence or absence of the instantaneous excitation frequency as anisolation criterion.

In an embodiment, the signal processor is configured with aninstantaneous frequency estimator to obtain an envelope of receivedsignal minus excitation signal from the undamped ultrasonic transceiverand to use a global phase and local slopes thereof as an estimate of theinstantaneous frequency, and further comprising an isolator unit forisolating signal segments whose instantaneous frequency approaches thecharacteristic frequency as segments containing primary echoes.

An embodiment may comprise a window unit for windowing the receivedsignal using a windowing length chosen to provide windows with anexpectation of a single primary echo.

In an embodiment, the signal processor is further configured to find apoint of appearance of a primary echo in a received signal bysuccessively dividing the curve and fitting to a linear functions andcalculating a point at which a corresponding error function isminimized.

An embodiment may be configured with a location unit to determine adistance to a first feature wall from the point of appearance.

In an embodiment, the location unit is configured to use a second pointof appearance of a further primary echo to determine a distance to asecond feature wall, the signal processor further comprising amonitoring unit for monitoring a distance between the first feature walland the second feature wall as an indicator of ablation progress.

In an embodiment, the signal processor comprising a convolution unit forconvolving an excitation signal with the received signal to carry outthe isolation of the primary echo.

In an embodiment, the signal processor comprises a Fourier componentanalyzer for isolating segments having a principle Fourier componentwhich corresponds to a body-characteristic frequency.

In an embodiment, the signal processor comprises a coherent summationunit for carrying out data summation such as to preserve amplitude andshift signals to a same phase.

In an embodiment, the coherent summation unit is configured to performcoherent summation, the coherent summation comprising building anauxiliary matrix of phase weights, making a Hilbert transform andmultiplying to bring all the signal to the same phase, therewith tocreate an in-phase sum.

An embodiment may comprise a reference subtracting unit configured tosubtract a reference from the transceiver signal by averaging severalsignal samples to eliminate unstable components.

According to a second aspect of the present invention there is providedan ultrasonic transceiver method for intracorporeal use, the methodcomprising:

placing an undamped ultrasonic transceiver in a confined intracorporealspace, the transceiver having a characteristic frequency

-   exciting the transceiver at an instantaneous excitation frequency to    produce an ultrasonic ablation beam for ablating surrounding tissues    using ablation pulses;

at intervals between the ablation pulses providing monitoring excitationto elicit primary echo signals returning from surrounding tissues;

isolating the primary echo signals from ringing, secondary echoes andextraneous noise also received from the transceiver using presence orabsence of the instantaneous excitation frequency as an isolationcriterion.

In an embodiment, the isolation comprises obtaining an envelope ofreceived signal minus excitation signal from the undamped ultrasonictransceiver and using a global phase and local slopes as an estimate ofthe instantaneous frequency, and isolating those signal segments whosefrequency approaches the instantaneous excitation frequency.

An embodiment may comprise windowing the received signal using awindowing length chosen to provide windows with an expectation of asingle primary echo.

An embodiment may be configured to find a point of appearance of aprimary echo in a received signal by

successively dividing the curve,

fitting to a linear functions and

calculating a point at which a corresponding error function isminimized.

An embodiment may comprise determining a distance to a first featurewall from the point of appearance.

An embodiment may comprise using a second point of appearance of afurther primary echo to determine a distance to a second feature wall,and monitoring a distance between the first feature wall and the secondfeature wall as an indicator of ablation progress.

An embodiment may comprise convolving an excitation signal with thereceived signal to carry out the isolation of the primary echo.

An embodiment may comprise isolating segments having a principle Fouriercomponent which corresponds to a body characteristic frequency.

An embodiment may comprise carrying out coherent data summation such asto preserve amplitude and shift signals to a single phase.

In an embodiment, the coherent summation comprises:

making an auxiliary weights matrix evaluation;

carrying out a Hilbert transformation;

multiplication to bring all signals to the same phase; and

performing an in-phase summation.

According to a third aspect of the present invention there is providedan ultrasonic transceiver apparatus for intracorporeal use, theapparatus comprising:

an undamped ultrasonic transceiver for placing in a confinedintracorporeal space, the transceiver having a characteristic frequencyand for receiving excitation at the characteristic frequency to producean ultrasonic ablation beam for ablating surrounding tissues and beingfurther for receiving excitation by primary echo signals returning fromsurrounding tissues;

a signal processor connected to the transceiver configured to isolatethe primary echo signals from ringing, secondary echoes and extraneousnoise also received from the transceiver, the signal processor usingcorrelation with a body-characteristic frequency as an isolationcriterion.

In an embodiment, the body characteristic frequency may be pulse orbreathing rate.

In an embodiment, the signal processor is configured to obtain a powerspectrum of a signal extracted from the transceiver and to identify theprimary echoes from peaks in the power spectrum at thebody-characteristic frequency.

An embodiment may comprise a coherent summation unit or a convolutionunit or both.

According to a fourth aspect of the present invention there is provideda method of providing controlled thermal damage to a tissue, comprising:

identifying locations of boundary walls of the tissue;

applying energy to the tissue;

during the applying, monitoring changes in locations of the boundarywalls as indicators of the application of energy to the tissue, forexample thermal shrinkage of the tissue; and

controlling the thermal energy according to the effect, for example,thermal shrinkage.

In an embodiment, the monitoring and applying are carried out fromwithin a blood vessel, and/ or using ultrasonics.

In this fourth aspect of the invention, a known ablation device may beused in conjunction with an ultrasonic detector.

Unless otherwise defined, all technical and/or scientific terms usedherein have the same meaning as commonly understood by one of ordinaryskill in the art to which the invention pertains. Although methods andmaterials similar or equivalent to those described herein can be used inthe practice or testing of embodiments of the invention, exemplarymethods and/or materials are described below. In case of conflict, thepatent specification, including definitions, will control. In addition,the materials, methods, and examples are illustrative only and are notintended to be necessarily limiting.

Implementation of the method and/or system of embodiments of theinvention can involve performing or completing selected tasks manually,automatically, or a combination thereof. Moreover, according to actualinstrumentation and equipment of embodiments of the method and/or systemof the invention, several selected tasks could be implemented byhardware, by software or by firmware or by a combination thereof usingan operating system.

For example, hardware for performing selected tasks according toembodiments of the invention could be implemented as a chip or acircuit. As software, selected tasks according to embodiments of theinvention could be implemented as a plurality of software instructionsbeing executed by a computer using any suitable operating system. In anexemplary embodiment of the invention, one or more tasks according toexemplary embodiments of method and/or system as described herein areperformed by a data processor, such as a computing platform forexecuting a plurality of instructions. Optionally, the data processorincludes a volatile memory for storing instructions and/or data and/or anon-volatile storage, for example, a magnetic hard-disk and/or removablemedia, for storing instructions and/or data. Optionally, a networkconnection is provided as well. A display and/or a user input devicesuch as a keyboard or mouse are optionally provided as well.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the invention are herein described, by way ofexample only, with reference to the accompanying drawings. With specificreference now to the drawings in detail, it is stressed that theparticulars shown are by way of example and for purposes of illustrativediscussion of embodiments of the invention. In this regard, thedescription taken with the drawings makes apparent to those skilled inthe art how embodiments of the invention may be practiced.

In the drawings:

FIG. 1 is a simplified schematic block diagram illustrating ultrasonictransceiver apparatus according to an embodiment of the presentinvention;

FIG. 2 is a simplified graph showing a raw signal received from thetransceiver and the same signal after reference subtraction;

FIG. 3 is a simplified graph showing the signal of FIG. 2 afteraveraging and after coherent summation;

FIG. 4 is a simplified graph showing the raw signal after noisesubtraction and after double coherent summation and convolution;

FIGS. 5A-5B and 6-8 illustrate experimental data for a measured distanceof 0.19 mm;

FIGS. 9A-9B and 10-12 illustrate experimental data for a measureddistance of 1.19 mm;

FIGS. 13A-13B and 14-16 illustrate experimental data for a measureddistance of 1.69 mm;

FIGS. 17A-17B and 18-20 illustrate experimental data for a measureddistance of 2.19 mm;

FIGS. 21A-21B and 22-24 illustrate experimental data for a measureddistance of 2.69 mm;

FIGS. 25A-25B and 26-28 illustrate experimental data for a measureddistance of 3.19 mm;

FIGS. 29A-29B and 30-32 illustrate experimental data for a measureddistance of 3.69 mm;

FIGS. 33A-33B and 34-36 illustrate experimental data for a measureddistance of 4.19 mm;

FIGS. 37A-37B and 38-40 illustrate experimental data for a measureddistance of 4.69 mm;

FIGS. 41A-41B and 42-44 illustrate experimental data for a measureddistance of 5.19 mm;

FIG. 45 is a correlation graph derived from the experiments of FIGS. 5Ato 45, and indicating a very high correlation between actual andmeasured distances;

FIG. 46 is a simplified flow chart illustrating the process of coherentsummation according to an embodiment of the present invention;

FIG. 47 is a flow chart showing in greater detail the actual transceiverfrequency evaluation of FIG. 46;

FIG. 48 is a simplified graph illustrating division of envelope sectionsaccording to amplitude of the principle Fourier component matching thedevice resonance, and showing the selection of the sections having thehighest amplitude, according to an embodiment of the present invention;

FIG. 49 illustrates ringing following an excitation signal, with primaryechoes hidden in the ringing;

FIG. 50 shows sampling to obtain vectors, undamped ringing with echoesincluded, and a power spectrum graph at the pulse frequency, where peaksin the power spectrum graph indicate features in the primary echo; and

FIG. 51 is a simplified flow chart illustrating a procedure forobtaining the power spectrum graph of FIG. 50.

DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION

The present invention, in some embodiments thereof, relates to anultrasound transceiver, in particular for use in confined spaces such asblood vessel to carry out self-monitored ablation on surroundingtissues.

The problem is to detect the wall of an artery (or any other tissue)using an ultrasonic detector which must be undamped because damping willreduce efficiency.

In general the effectiveness of an ablation process can be determined bymeasuring the distance between outer and inner walls of a tissue beingablated. Specifically, as the tissue is ablated, the distance betweenthe outer and inner walls falls, so sequential monitoring of thedistance is a way of measuring the effectiveness of the ablationprocess.

Since the detector is undamped there is considerable noise due toringing. The ringing is present in the sensor and is also transmitted tothe tissues causing reflections of its own, all this interfering withthe signal it is intended to detect.

The issue is that the signal from the artery wall is drowned out bynoise and the ringing. We know however that the signal from the tissuewall has certain characteristics. The primary ringing tends to share amain frequency with the excitation signal, although the phase willdiffer depending on the distance Thus the frequency can be used todistinguish primary echoes from ringing and from secondary echoes aswell as general noise.

Once the primary echoes have been distinguished then it is possible tomonitor the thermic effectiveness of the treatment by locating the innerand outer walls and monitoring the change in distance between them.

More generally, detecting the primary echoes allows them to be used inan analysis of the entire artery wall tissue signal, and not only theface of the wall that is touching the blood.

Various methods are given in the disclosure for analyzing the primaryechoes.

Since the transceiver is a narrow band device, it irradiates mainly atits Eigen frequency. Furthermore, voltages generated by returnedpressure waves are again filtered by the transceiver, so that theprimary reflected signal is an almost pure harmonic oscillation, albeitwith variable amplitude. It is reasonable to expect that transient-onlyportions of the signal are characterized by different frequencies.

The present embodiments further include a means of providing controlledthermal damage to a tissue, by identifying locations of boundary wallsof the tissue, applying energy to the tissue, and monitoring changes inlocations of the boundary walls as indicators of the application ofenergy to the tissue, for example thermal shrinkage of the tissue; Thenthe application of the thermal energy can be controlled y according tothe detected effect.

In an embodiment, the monitoring and applying are carried out fromwithin a blood vessel, and/ or using ultrasonics. The above may becarried out using the transceiver described herein but alternatively, aknown ablation device may be used in conjunction with an ultrasonicdetector.

It is noted that the effect on the tissue is not necessarily shrinkage.It may instead be thickening due to heating or to some physiologicalresponse. Nevertheless it is the change in size or shape of the tissue,as determined by changes in tissue wall location determined bynon-imaging or imaging of tissue, that is used to control the process.

The present embodiments relate to non-focused ultrasound, or to focusedultrasound or to r.f. based systems.

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not necessarily limited in itsapplication to the details of construction and the arrangement of thecomponents and/or methods set forth in the following description and/orillustrated in the drawings and/or the Examples. The invention iscapable of other embodiments or of being practiced or carried out invarious ways.

Referring now to the drawings, FIG. 1 illustrates an ultrasonictransceiver apparatus 10 primarily intended for intracorporeal use,according to one embodiment of the present invention. A transceiver 12is designed to be injected into blood vessels and like confined spaceswithin the body, particularly with a view to flowing towards locationswhere tissue ablation is required, and to direct ultrasound energytowards the tissue to be ablated. In between ablations the transceiveris excited with monitoring signals to use ultrasound energy for adifferent purpose, the monitoring of the surrounding tissues to makesure that the ablation is being carried out effectively.

As mentioned in the background ablation takes place in the body, whereheating of the wrong tissues is undesirable. Thus the transceiver isrequired to be as efficient as possible and thus to be undamped.Monitoring however requires a damped transceiver since reading theechoes is conventionally only possible once the excitations have dieddown. Using different transceiver surfaces for monitoring and ablationrespectively is also not ideal since it is difficult to guarantee thatthe tissue being monitored is the same as the tissue being ablated.

The present embodiments thus use a single undamped transceiver for bothmonitoring and ablation and methods and a structure are provided forisolating the primary reflections from ringing due to the excitationsignal and due to secondary reflections from the ringing as well asassorted other noise. The undamped ultrasonic transceiver may be anarrow band transceiver having a characteristic excitation frequency sothat all excitations are at that characteristic frequency. The primaryechoes tend to share the excitation frequency, albeit at variable phasesand amplitudes, whereas general ringing and tend to be at other gfrequencies.

A signal processor 14 is connected to the transceiver 12 to isolate theprimary echo signals from the ringing, secondary echoes and extraneousnoise also received from the transceiver. The signal processor usespresence or absence of the characteristic frequency as an isolationcriterion.

An excitation unit 16 provides an excitation signal for the transceiver.The excitation may be high power for the ablation or low power formonitoring.

The signal processor may have reference subtraction unit 18 whichsubtracts the excitation signal, from the transceiver 12.

As will be discussed in greater detail below, the size of the segmentsmay be chosen so that only one primary echo need be found per segment.Otherwise the presence of multiple reflections makes analysis moredifficult. The distances between tissue walls are known in general termsso segment lengths are chosen to represent distances smaller than thedistance between walls. Thus a window unit 22 windows the receivedsignal using a windowing length chosen to provide segments with anexpectation of a single primary echo.

There are a number of alternative ways in which the segment isolator canfind primary echoes.

A convolution unit 20 convolves an excitation signal with the receivedsignal to isolate only those waveforms having a high coherence with theexcitation signals.

Following convolution is coherent summation. A coherent summation unit27 carries out data summation such as to preserve amplitude and shiftsignals to the same phase. The coherent summation unit may build anauxiliary matrix and then multiplies it with previously carried outHilbert transform to shift the obtained complex analytic signals to thesame phase and to make in-phase (coherent) summation Coherent summationis discussed in greater detail below.

Although it is possible to use convolution alone or coherent summationalone it is also possible to combine operation of the above units. FIG.4 below discusses double coherent summation and convolution as a way toobtain the primary echoes.

The above processes may involve obtaining a reference signal, based onthe actual excitation. Reference subtracting unit 18 may subtract areference from the transceiver signal by averaging several signalsamples (particular records), simply to eliminate unstable componentsand obtain the actual excitation. Optionally, one can use as a referencea separate signal where echoes do not arrive until after the ringingdies out. That is, a part of the signal where the ringing hasdisappeared but echo has not appeared yet can be used as a reference. Aseparate reference signal is known to be free of echoes in the region ofinterest. Thus, its subtraction from the particular signal reliablyprovides us with almost pure echo, except for the noise component.

Finally, after double coherent summation and convolution, an envelope isobtained.

With the envelope, the signal processor may find a point of appearanceof a primary echo in the segment by successively dividing the envelopecurve while fitting to a linear function and calculating a point atwhich a corresponding error function is minimized. Such a process isillustrated in FIG. 6 below.

The point of error minimization is most probably the point of onset ofthe primary echo and indicates the distance to a structure such as atissue wall. Such an error minimization is shown in FIG. 7 discussedbelow. A location unit 24 determines the distance to a first featurewall based on the point of appearance.

The location unit may then use a second point of appearance of a furtherprimary echo to determine a distance to a second feature wall. A processmonitoring unit then calculates and monitors the distance between thefirst feature wall and the second feature wall as an indicator ofablation progress. As the ablation progresses the wall typically shrinksas the tissue dries out.

An alternative to the above is Fourier component analysis, based not onthe excitation frequency of the transceiver, but on frequenciescharacteristic of the body, such as pulse rate or breathing rate.

Using Fourier component analysis, the signal processor uses correlationwith a body-characteristic frequency as an isolation criterion. One wayof doing this is to obtain a power spectrum of the signal extracted fromthe transceiver and to identify the primary echoes from peaks in thepower spectrum at the body-characteristic frequency.

The Fourier component analysis may be used instead of or in addition tocoherent summation and to convolution using a convolution unit.

The above processes are now considered in greater detail.

As mentioned, an envelope of the oscillating signal is obtained, and aglobal phase and its local slopes are used as estimations ofinstantaneous frequency (actually, instantaneous frequency is localslope itself). Also instantaneous frequency is estimated from the entiresignal in analytical form, as time derivative of phase. The appearanceof a primary reflection signal is then recognized within the overallenvelope as a transition at an instantaneous frequency of the signal.The transceiver frequency remains constant.

The Hilbert transform is carried out on a residual signal, afterreference subtraction. The results are used for coherent summation oforiginal signals. signal construction the Hilbert transform effectivelycreates a signal shifted by a quarter of a period with respect to theoriginal signal. For a true harmonic signal, the phase of the obtainedcomplex analytic signal—see FIG. 5B, grows linearly in time. If a newharmonic component appears in the signal, it may be revealed by a changeof the phase-time curve slope.

The appearance of a new harmonic component may be taken as theindication of a new feature that is being detected, say the near surfaceof a tissue in question, and a second new harmonic component mayindicate the far surface of the tissue in question artery wall.

Finding the Primary Echo

To find the instant of new component appearance, an algorithm forminimum total error may be used. As illustrated in FIG. 6, the entirecurve may be divided in two parts and each part may then be fitted to alinear function. We consider double fitting of the entire error as afunction of the separation point. The assumption is that for the trueseparation point between two frequency regions the overall error reachesits minimum. In general, a change in best slope indicates the appearanceof a new event. The point where the new harmonic is added turns out tobe derivable from the error function. The overall error of the two curvefitting operations is a function of the point of appearance of the newharmonic, and minimization of the overall error may indicate the point.FIG. 7 illustrates such an error function with a clear minimizationpoint.

An issue arises in that the result of the above-described procedure issensitive to the length of the data subjected to analysis. Theabove-described procedure works best when a stretch of data containsjust a single point at which a new harmonic is added. But, in real lifethere are more, and surfaces have inner and outer walls which each giverise to separate reflections and there are second order reflections andalso noise. Thus, the presence of a second reflection or say a longportion of noise, negatively affects the accuracy. The problem may besolved by windowing, analyzing the received data in short chunks, aswill be explained in greater detail below.

Correlation Approach

The excitation voltage generates mechanical oscillations in thetransceiver, which irradiate pressure waves into surrounding media.Return pressure waves generate a reflection voltage signal. It isreasonable to expect that the reflection retains at least a partialcorrelation with the excitation voltage. An alternative or additionalapproach is just to convolve part of the excitation signal with the restof the data. The reflection may thus be amplified according to thecorrelation and become more recognizable.

Reference is now made to FIG. 8, which illustrates local slopes of aphase-distance curve. Over the full phase-time (distance) curve one canevaluate local standard statistical parameter correlation coefficients.For perfectly linear dependence between two values the correlationcoefficient is ±1. For noisy data or for nonlinear dependence, theabsolute value is less than 1. A stable and significant decrease ofcorrelation coefficient serves as a reliable sign of useless noisy dataand allows the irrelevant signal to be excluded.

The use of correlation coefficients can thus be used to find parts ofthe data where there are strong echoes and ignore parts where suchstrong echoes are absent.

Experimental Calibration

The present embodiments were applied experimentally to estimate distancewithin the interval 0.19-5.19 mm.

Two types of experiments were carried out, in vitro and in vivo.

In in vitro experiments, it is possible to control distance betweentransceiver and reflective object (metal plate) by means of apositioning device with a ruler and set it in parallel to thetransceiver surface. Accuracy of 0.1 mm is available. In the following,in vitro experimentation is used to generate a calibration curve betweenmeasured and actual distances. That is, to the experiment finds slopeand intercept. The slope is expected to be close to 1. The interceptallows for elimination of systematic error in the ruler reading. Indeed,the intercept is not the interesting point, although technicallynecessary. The indication of evaluation correctness is that theevaluated points lie on a straight line with a slope close to 1.

Several remote points with reliable separation between transient andreflection were tested and an offset (the above-referred to intercept)was estimated. A true distance can then be used which is the sum of theruler reading and offset.

Sample Equivalents

In the time domain, the sample equivalent is a reciprocal samplingfrequency. For 800 MHz, it is 1.25 ns.

In the space domain, the sample equivalent is half of the samplingperiod multiplied by sound velocity. Its interpolated value at 23° C. is1489 m/s. Thus, a distance equivalent is 9.306·10−4 mm.

FIG. 2 upper part presents a raw signal and the lower part shows thesame after reference subtraction. FIG. 3 upper part shows averaging andgives a ratio of maximum amplitude over noise standard deviation to be5.86. The signal shown represents results of averaging 11 particularsignals. The lower part of FIG. 3 shows coherent summation and managesto achieve a ratio of maximum amplitude over noise standard deviation of10.2, nearly double the level achieved by averaging.

FIG. 4 presents result of repeated coherent summation of 23 groups(total 11×23=253 processed signals of 256) and convolution with aresonant window.

According to the above considerations, the best improvement of SNR for11 particular signals is 3.32. It follows from FIG. 3 that the realimprovement is 1.74. Specifically, the improvement is the ratio of SNRsafter and before processing. SNR after processing is 10.2, and SNRbefore is 5.96. SNR after processing is greater.

FIGS. 5A-8 are as discussed above and illustrate estimated and realdistance results for distances of 0.19 mm or 0.69 mm. FIG. 5A shows theraw signal and FIG. 5B shows the corresponding analytical signal. FIG. 6shows the curve fitting to find the point at which the primary echocomponent appears. FIG. 7 illustrates the error function and FIG. 8illustrates the full phase-distance curve from which local slopes can beobtained.

FIGS. 9A-12 illustrate corresponding estimated and real distance datafor a distance of 1.19 mm. FIG. 9A shows the raw signal and FIG. 9Bshows the corresponding analytical signal. FIG. 10 shows the fitting.FIG. 11 illustrates the error function and FIG. 12 illustrates the fullphase-distance curve.

FIGS. 13A-16 illustrate results for a distance of 1.69 mm. FIG. 13Ashows the raw signal and FIG. 13B shows the corresponding analyticalsignal. FIG. 14 shows the fitting. FIG. 15 illustrates the errorfunction and FIG. 16 illustrates the full phase-distance curve.

FIGS. 17-20 illustrate results for a distance of 2.19 mm. FIG. 17A showsthe raw signal and FIG. 17B shows the corresponding analytical signal.FIG. 18 shows the fitting. FIG. 19 illustrates the error function andFIG. 20 illustrates the full phase-distance curve.

FIGS. 21A-24 illustrate results for a distance of 2.69 mm. FIG. 21Ashows the raw signal and FIG. 21B shows the corresponding analyticalsignal. FIG. 22 shows the fitting. FIG. 23 illustrates the errorfunction and FIG. 24 illustrates the full phase-distance curve.

FIGS. 25A-28 illustrate results for a distance of 3.19 mm. FIG. 25Ashows the raw signal and FIG. 25B shows the corresponding analyticalsignal. FIG. 26 shows the fitting. FIG. 27 illustrates the errorfunction and FIG. 28 illustrates the full phase-distance curve.

FIGS. 29A-32 illustrate results for 3.69 mm. FIG. 29A shows the rawsignal and FIG. 29B shows the corresponding analytical signal. FIG. 30shows the fitting. FIG. 31 illustrates the error function and FIG. 32illustrates the full phase-distance curve.

FIGS. 33A-36 illustrate results for a distance of 4.19 mm. FIG. 33Ashows the raw signal and FIG. 33B shows the corresponding analyticalsignal. FIG. 34 shows the fitting. FIG. 35 illustrates the errorfunction and FIG. 36 illustrates the full phase-distance curve.

FIGS. 37A-40 illustrate results for a distance of 4.69 mm. FIG. 37Ashows the raw signal and FIG. 37B shows the corresponding analyticalsignal. FIG. 38 shows the fitting. FIG. 39 illustrates the errorfunction and FIG. 40 illustrates the full phase-distance curve.

FIGS. 41A-44 illustrate the results for a distance of 5.19 mm. FIG. 41Ashows the raw signal and FIG. 41B shows the corresponding analyticalsignal. FIG. 42 shows the fitting. FIG. 43 illustrates the errorfunction and FIG. 44 illustrates the full phase-distance curve.

Each estimation is average on 6 sets of 5 times averaged data. Exceptfor one point (4.69 mm), standard deviation is not significant. Withoutliers excluded, estimation in this point gives 4.61 mm. Absolute RMSerror is 0.24 mm.

Referring now to FIG. 45, a correlation curve is obtained by fittingmeasured results to actual distances. A straight line is fitted to thedata points, with slope 0.89 and intercept 0.18 mm. Correlation of knownand estimated distances is high, and the observed correlationcoefficient is 0.9954.

General Description of the Problem and Requirements

The procedure is now considered in more detail. It is required toestimate distance from a transceiver surface to artery wall. The rangeof interest lies between 1 . . . 5 mm. Allowed error is 0.2 mm.Processing time is not intended to exceed 2 s.

Features and Difficulties

As mentioned, there is uncertainty in mutual orientation of and arterywall, known as angular uncertainty.

The reflected signal is typically small. That is, first, it is corruptedby noise in general and, second, is distorted by a residual excitationsignal, otherwise known as ringing. As mentioned, the ringing canproduce second order effects such as echoes of the ringing. Thereflected signal is a superposition of multiple similar signals eachproduced by its own source the source. That is, the shape of reflectedsignal may vary depending on the angular position of the transceiver.

Within the same in vivo experiment, it is possible that a giventransceiver surface may be polluted with coagulated blood, causingchanges in the characteristic frequency.

Fortunately, in general, applied transceivers are narrow band devicesand reflected signals are close to harmonic, however, with variableamplitude. This allows for efficient signal isolation.

Data Averaging

Data are built as a matrix, whose columns represent a reaction wherein asingle record maps to a single pulse. Then, columns of the records aredivided into groups on which averaging is carried out. Following this,each column is processed on the basis of the averaged groups and overallresults are averaged. In the matrix, row size may represent the totalnumber of points recorded in a single echo response, and the columnnumbers may represent the total number of echo responses which wasrecorded.

As an example of the above, in in vivo experiments which were carriedout, datasets were obtained. Each dataset consisted of 1000 records,each having about 12000 samples, and corresponding to a duration ofabout 0.15 μs. The PRF was 400 Hz, therefore total duration of theexperiment was 2.5 s. Sometimes only the first 256 records were used forprocessing. The first 1000 samples (about 0.95 mm) were excluded fromprocessing. Data length was restricted to 2¹³=8192 samples (about 8 mm)to accelerate Fourier transforms. The best case occurs when the numberof samples is a power of 2. However, FFT performance it is also good ifit is the product of several primes. Optional fine tuning of data lengthis also carried out.

Reference Subtraction

At small distances the entire signal is a superposition of the residualtransceiver signal, the free reaction, and the reflected transceiversignal. An apparent solution would be to subtract the pure excitationsignal without reflections from the superposition to obtain thereflection, but this fails to take into account noise or second ordereffects.

Thus in an embodiment, a reference signal is obtained by averaging ofseveral records. Presumably, due to instabilities, phases of reflectedsignals are randomly different from each other, and averaging weakensthis component. Contrary, the excitation pulse is taken as stable soaveraging does not cause any deterioration. Thus, in principle theunstable component is more or less efficiently eliminated from theprocessed signal.

Noise Level

Noise levels may be estimated from a remote part of a record wherereflections are not expected.

Coherent Summation

Having identified the most relevant windows, the procedure thenidentifies the primary echo based on coherent summation. Consider a setof signals, which are expected to be similar to each other. Ideally, allsignals would be the same, except for the noise component. Thussummation of N such signals increases the entire signal N times whilenoise increases only √{square root over (N)} times. As a result, the SNRimprovement is √{square root over (N)} times. In practice, however, dueto random phase shifts the described procedure becomes less efficient.This can be solved by means of a Hilbert transform producing a complexanalytic signal, in which each signal can be assigned a phase, which,for a true harmonic signal would be the true phase. Thus, phase isdefined conventionally as for a usual complex number:

$\begin{matrix}{\phi = {{arc}\; \tan \frac{({Hx})}{\Re ({Hx})}}} & (1)\end{matrix}$

Let the k-th signal generate Hilbert phases α^((k)). We wish to find atransform which preserves amplitude and brings all signals of the set tothe same phase or, at least, minimizes phase deviation of each signalfrom an averaged phase α. Let z=x+ty be the complex analytic signalgenerated by the Hilbert transform. Thus new signals are created asfollows:

u=x cos γ−y sin γ, v=x sin γ+y cos γ

which may be expressed as,

w=z exp(i γ).

Such a transform preserves amplitude

√{square root over (u ² +v ²)}=√{square root over (x ² +y ²)}=α.

We wish to minimize (weighted) error

$ɛ = {\sum\limits_{k}\; {W^{(k)}{{{\frac{z^{(k)}\exp \mspace{11mu} \; \gamma_{k}}{a^{(k)}} - {\exp \mspace{11mu} \overset{\_}{\alpha}}}}^{2}.}}}$

For simplicity, summation over elements of each signal is omitted. Inequivalent form,

$ɛ = {{2{\sum\limits_{k}\; W^{(k)}}} - {2\Re {\sum\limits_{k}\; {\frac{W^{(k)}}{a^{(k)}}{\left( {z^{*{(k)}}{\exp \left( {\overset{\_}{\alpha} - {\; \gamma_{k}}} \right)}} \right).}}}}}$

Since we wish to have the best agreement for signal but not for noise,weights may be chosen proportional to amplitude. Angles providingminimum error for the k-th signal are found from the equation

$\frac{\partial ɛ}{\partial\gamma_{k}} = {{2{\Re \left( {{iz}^{*{(k)}}{\exp \left( {{\overset{\_}{\alpha}} - {\; \gamma_{k}}} \right)}} \right)}} = 0.}$

In matrix notation,

$\begin{matrix}{{E_{k} = {{\exp \left( {\; \gamma_{k}} \right)} = \frac{z^{H{(k)}}e}{{z^{H{(k)}}e}}}},{e = {{\exp \left( {\overset{\_}{\alpha}} \right)}.}}} & (2)\end{matrix}$

The denominator provides proper normalization such that entries of theabove weights matrix are true phase exponents. The upper index H meansmatrix Hermit conjugation (or Hermitian conjugation), which istransposition together with complex conjugation. The signal aftercoherent summation is simply

x_(coh)=

(zE).  (3)

A first factor in equation 3 is the analytic signal corresponding to theentire set of signals. A second factor is an auxiliary phase weightmatrix, which provides summation with proper phases.

Step-Wise Algorithm Description

Reference is now made to FIG. 46 which illustrates a possible procedure.

S1 involves carrying out a Hilbert transform on the entire raw data:

z=Hx=x+iy

to obtain an analytic signal. S2: Reference signal construction uses

$z_{ref} = {\frac{1}{N}{\sum\limits_{k = 1}^{N}\; {z^{(k)}.}}}$

S3: Reference subtraction. In an embodiment, individual subtraction isused. That is to say, for each set of data, the reference signal ismultiplied by an individual scaling factor which is close to 1. Thescaling factor is evaluated using the initial portion of data (below 1mm). Individual scaling factors provide the least RMS error beingapplied to a particular signal, but to a portion, which is not involvedin further processing. In the present embodiments, these values arepropagated to the remaining portion of the data for more efficientsubtraction of the excitation signal.

Z→Z−Z_(ref).

S4: Averaging. Each group of columns is averaged. Members of the groupparticipate in the coherent summation procedure. In an embodiment, anoptimal arrangement of groups is used. In this arrangement, within eachgroup, phases are closer to each other.

S5: carrying out a phase evaluation, again based on formula (1).

S6: Construction of particular coherent signal. See formulae (2) and(3).

S7: Actual transceiver frequency evaluation. S8 is shown in greaterdetail in FIG. 47.

In S7.1, phases of all particular coherent signals are averaged.

In S7.2, the entire phase curve is divided into small pieces (about 100samples), and for each piece a local strip and a normalized linearfitting error are evaluated. Optionally, local slopes are obtained onoverlapping pieces. An instantaneous frequency may be found at eachpoint. Alternatively, instantaneous frequencies may be obtained by usageof a formally exact definition of frequency as a phase time derivative.A Hilbert transform of the derivative may be involved.

In S7.3, for some of the smallest errors (about 10), corresponding localslopes are averaged, to give the actual transceiver frequency.

In S7.4, the actual transceiver frequency is used for resonant windowconstruction. Namely,

w _(res)(t)=w(t)exp(2πift).

A first factor in the above is a standard windowing factor (Hanning,Gauss, Kaiser, etc.). A size of the window may correspond to 2 periods.

Returning now to FIG. 46 and in S8: Convolution with a resonant windowis shown. An absolute value of the convolved complex signal (amplitude,or envelope) is used for further analysis.

S9: Thresholds and distance evaluation. It must be emphasized that thereis no rigorous rule to choose a threshold for a reflected signalappearance because its shape may change even within the same experiment.Besides, the origin of a transceiver transient reaction may beinherently small and therefore to all intents and purposes areinvisible. However, one can set an empirically based value of athreshold and consider reflection appearance as the point at which asignal firstly falls below the threshold, starting from its maximum.Since signal growth usually occurs rapidly, the result must not besensitive to the threshold variations or uncertainty. In the currentversion of the algorithm, two alternate threshold estimations are used:

1. Noise level. A remote section of the data where reflections are notexpected is used to estimate noise level. The threshold may then be setas

T₁=k₁

S_(noise only)

.

Since the final signal is amplitude (envelope), noise level is estimatedas its mean. 2. Peak height. A threshold is

T₂=k₂s_(max).

This second definition is used when the noise level is found to be toohigh. Sometimes extremely stretched signals are observed. Dimensionlessfactors in these formulas are established based on visual analysis ofthe processed signal and may be subjected to fine tuning.

It is possible that reflected signals start at a relatively high leveland do not fall below a threshold. Such would be an indication that theorigin is closer than 0.95 mm.

EXAMPLES

Let us define SNR as a ratio of the largest amplitude of a reflectedsignal to SD evaluated in the region of pure noise:

${SNR} = {\frac{\max (A)}{\sigma_{noise}}.}$

Considering again FIGS. 1 to 3, FIG. 1 presents a raw signal and thesame after reference subtraction. FIG. 2 presents results of justaveraging 11 particular signals vs. coherent summation. FIG. 3 presentsthe result of repeated coherent summation of 23 groups (total 11×23=253processed signals of 256) and convolution with a resonant window.

According to the above considerations, the best improvement of SNR for11 particular signals is 3.32. It follows from FIG. 3 that the realimprovement is 1.74 based on the SNR values shown and as discussedabove.

Fourier Component Approach

Reference is now made to FIGS. 48-51 which illustrate an approach basedon grouping of samples according to relationships between the echosignals, body frequencies such as pulse or breathing, and the originalexcitation. The relative position of tissues within the body changesover short times due to breathing and blood pulsation. Ultrasonic echoesfrom tissues obey such a periodicity whereas ringing artifacts and othertypes of noises do not. This is specifically true for ultrasonic echoesmeasured intravascularly. Consequently, the existence of suchperiodicity can be utilized to separate between tissue reflection andother noise sources in echoes with low signal to noise ratio.

In order to use this information, one must first estimate theperiodicity of the echo reflections over consecutive measurements. Thisperiodicity reflects both the movement of the catheter as well as themovement of the tissue.

Several ways can be used to identify the movement profiles of tissue inecho signals. One way is to obtain segments of the echo signals in whicha strong reflection is observed. This method is described as follows.FIG. 48 (upper left part) shows the echo response of consecutive trialsas a function of distance. A clear periodic change in the echo responsefrom the tissue is observed. Since a narrow band transceiver is used, itis possible to quantify the reflection intensity by calculating theFourier component of the central transceiver frequency Fc in consecutivetime windows (Fi) of duration D and overlap OL:

$F_{i} = \sqrt{\begin{matrix}{\left( {\int{{\sin \left( {2\pi \; F_{c}t} \right)}{X(t)}{t}}} \right)^{2} +} \\\left( {\int{{\cos \left( {2\pi \; F_{c}t} \right)}{X(t)}{t}}} \right)^{2}\end{matrix}}$

The response profile in the time domain is illustrated in the lower partof FIG. 48. From the profile, ND windows with the highest values of thecentral frequency Fourier component are chosen for cross-correlationanalysis. The relevant windows are marked by red circles in the lowerpart of FIG. 48. Using cross correlation analysis between consecutivetrials, it is possible to identify the shifts in distance between thetissue and the catheter, as illustrated in the right part of FIG. 48.Once the movement profile of the tissue is obtained for many tissuessamples, this movement profiles can be used to estimate thecharacteristic frequencies related to breathing and blood pulsation.

Then, the characteristic frequency profiles due to tissue movement areused to separate between the characteristic frequencies and ringingartifacts and other noise signals. In greater detail, owing to the bloodpressure, the catheter moves at similar frequencies to the bloodpressure, as illustrated in the right part of FIG. 48. On the other handthe echo readings due to catheter ringing as well as other noises arenot influenced by the blood pulsation. Thus, the echo signal distancecan be distinguished from the background by the fact that it movesbackward and forward in correspondence to the blood pulsation. Thus,separating the signals that show the periodicity of the blood pressuremovement from other noises may help to identify primary tissue echoeseven though their signal to noise ratio is low. This allows accuratedetection of the distance to the tissue wall.

To do so, consecutive echo signals are first collected and the powerspectrum is calculated for every point in the echo along consecutivetrials. This is illustrated in FIG. 49 which shows the power spectrum ofeach point of the echo. Each point corresponds to a different distance.The power spectrum intensity is color coded from blue to red. Next, theratio between the frequency components of breathing and blood pulsation,which were estimated as described earlier, and the frequency componentsof noise and ringing are calculated for each point of the echo signal.The response ratio is defined as the reflection profile and is used toseparate tissue reflections from other noises.

The results of such a process are illustrated in FIG. 50. The upper partof FIG. 50 shows 256 consecutive echo trials as a function of distance.Echo intensity is color coded from blue to red. The middle part showsfive superimposed individual trials. Clearly, the variability profilebetween different signals during the ringing is different from thevariability during reflections. These changes in variability arereflected in the power spectrum over consecutive echoes. The lower partof FIG. 50 is a plot of the reflection profile (in blue). A smoothedreflection profile (green) is obtained using a median filter. A rise inthis profile indicates reflections from the tissue due to increase infrequencies associated with blood pressure and breathing. Finally, thedistance from the artery can be identified using a threshold crossing ofthe smoothed reflection intensity function, the green line in FIG. 50.The threshold is automatically detected by analyzing the spectrum ofpoints with no tissue reflections, which points are detected using theirlow kurtosis values.

Referring now to FIG. 51, a summary of the procedure to achieve theabove is as follows: Firstly, in 51.1, the power spectrum analysis overthe same sample in consecutive echos is performed. In the given example(FIG. 50), the echo signal is recorded 256 times, to form 256 records ata sampling frequency of 400 Hz. Within each record, 10000 points aresampled at 800 MHz.

A power spectrum is calculated for each point over the 256 consecutivetrials, giving 10000 power spectrum graphs each being 256 samples inlength. Following the calculation of the power spectra, the reflectionintensity of every point is calculated by using the freq. ratio betweenthe blood pressure frequencies and noise frequencies at each point(S51.2). Tissue reflections are identified by their high reflectionintensity (S51.3). The tissue wall location is identified using anautomatically detected threshold.

Although in the above, the Fourier component approach is presented as analternative to coherent summation and convolution, it may also be usedtogether with either or both of coherent summation and convolution toproduce improved results.

Rationale

Reliable measurement may require transceiver—tissue boundary distanceswhich are greater than 1 mm and up to 5 mm.

Signal acquisition is performed at 800 MHz/14 bit, and with a suitableoscilloscope such as that produced by Agilent™, the sampling rate may beincreased to 2 GHz, but at a modest 8 bit resolution. With transceivercharacteristic frequency about 10 and 20 MHz, time resolution is 80-40samples per period.

Current implementations use PRF 400 Hz, which for 1000 repetitionsconstitutes a measurement duration of 2.5 s. This is enough to observeat least one entire period of heart pulse and therefore a full cycle ofan artery dilating and shrinking. This linkage may facilitate distancemeasurement. In the experiments above there was processing of only 256repetitions, using about 9000 samples each. At a PRF of 80 kHz, theentire measurement may take 3.2 ms. This time is too short forsignificant changes in mutual orientation of transceiver and tissueboundary, or distance between the transceiver and tissue boundary, andthus is expected to improve accuracy. In addition, a long acquisitionprocess provides mutual incoherence of reflected signals which weakensthe reflected signal level in the reference.

Currently the transceivers are narrow band. Their advantage is that themain component of their signal is at the Eigen frequency at differentexcitation shapes. Reflected signal appearance is seen clearly using theHilbert transform and direct evaluation of local slopes of thephase-time curve. Another possibility is to create a Hilbert transformbased instantaneous frequency. A special excitation shape may shortentransient reaction and therefore decrease the low measurement limit.

Due to averaging, a more stable excitation signal is summed almostcoherently, while the reflected signal due to random phase variations isweakened with respect to the excitation signal. Thus simple averagingpermits the creation of a reference which can be subtracted from aparticular signal to almost eliminate the effects of the excitationsignal. For better suppression of a (possible) narrow residual spike alow pass filter (LPF) may be applied. As a further point, due tomalfunction or the like of an acquisition device, a portion of thesignal that is intended for processing may sometimes contain highfrequency oscillations between saturation levels. These oscillations arenot the same for all particular signals and thus simple averaging is notefficient. In an embodiment, these spikes are identified, cut off, andinterpolated with further optional filtration.

Although an Eigen frequency is supplied for the transceiver, it isreasonable to make a data-based estimation. Due to variable amplitude,an instantaneous frequency may change within certain limits even for apure single-frequency signal. Thus, the frequency may be found fromlocal slopes, which fall within predefined tolerances of the transceiver(say, ±15%) and provide the smallest linear fitting errors. Parts of thedata subjected to coherent summation are those which correspond topoints which have a frequency that is close to the nominal transceiverfrequency. Optionally, all entries of the instantaneous frequency matrixcan be subjected to a logical operation which returns true if thefrequency falls within the predetermined tolerances and false otherwise.A summation over the rows indicates how suitable a current point may befor coherent summation, and then a predetermined number of the pointswith largest sums are taken for coherent summation. A resonant windowmay then amplify true frequency components.

Phase correction of particular signals, that is bringing all signals ofinterest to the same phase, allows improvement in the SNR. Coherentsummation is carried out twice: first, within groups of signals, forexample eleven signals may be placed in a group; and then secondly overthe groups themselves. Thus in an example of 23 groups, each with elevensignals, the summation would involve 11×23=253 signals. The obtainedsignal is convolved with a resonant window. Again, as discussedhereinabove, for better efficiency, groups may be arranged such that,within each group mutual closeness is better.

The above processing algorithm may provide a rapidly growing envelopesignal. Since the slope is large, an error due to uncertainty of thethreshold (signal appearance above noise) decreases.

The results of processing may show some cases of multiple reflections,which do not look as though they are reflections from the same object.It is likely that there are different objects with different reflectingproperties. However the algorithm of the present embodiments provides arelative height and a length with each peak. The present embodiments mayadditionally help to classify/recognize detected objects.

It is expected that during the life of a patent maturing from thisapplication many relevant ultrasound devices and transceivers will bedeveloped and the scope of the corresponding terms used herein isintended to include all such new technologies a priori.

The terms “comprises”, “comprising”, “includes”, “including”, “having”and their conjugates mean “including but not limited to”.

The term “consisting of means “including and limited to”.

As used herein, the singular form “a”, “an” and “the” include pluralreferences unless the context clearly dictates otherwise.

Whenever a numerical range is indicated herein, it is meant to includeany cited numeral (fractional or integral) within the indicated range.The phrases “ranging/ranges between” a first indicate number and asecond indicate number and “ranging/ranges from” a first indicate number“to” a second indicate number are used herein interchangeably and aremeant to include the first and second indicated numbers and all thefractional and integral numerals therebetween.

It is appreciated that certain features of the invention, which are, forclarity, described in the context of separate embodiments, may also beprovided in combination in a single embodiment. Conversely, variousfeatures of the invention, which are, for brevity, described in thecontext of a single embodiment, may also be provided separately or inany suitable subcombination or as suitable in any other describedembodiment of the invention. Certain features described in the contextof various embodiments are not to be considered essential features ofthose embodiments, unless the embodiment is inoperative without thoseelements.

Although the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims.

All publications, patents and patent applications mentioned in thisspecification are herein incorporated in their entirety by referenceinto the specification, to the same extent as if each individualpublication, patent or patent application was specifically andindividually indicated to be incorporated herein by reference. Inaddition, citation or identification of any reference in thisapplication shall not be construed as an admission that such referenceis available as prior art to the present invention. To the extent thatsection headings are used, they should not be construed as necessarilylimiting.

1. An ultrasonic transceiver apparatus for intracorporeal use, theapparatus comprising: an undamped ultrasonic transceiver for placing ina confined intracorporeal space, the transceiver having an instantaneousexcitation frequency and for receiving excitation at said excitationfrequency to produce an ultrasonic ablation beam for ablatingsurrounding tissues and being further for receiving excitation byprimary echo signals returning from surrounding tissues; a signalprocessor connected to said transceiver configured to isolate saidprimary echo signals from ringing, secondary echoes and extraneous noisealso received from said transceiver, said signal processor usingpresence or absence of said instantaneous excitation frequency as anisolation criterion.
 2. The ultrasonic transceiver apparatus of claim 1,wherein said signal processor is configured with an instantaneousfrequency estimator to obtain an envelope of received signal minusexcitation signal from said undamped ultrasonic transceiver and to use aglobal phase and local slopes thereof as an estimate of saidinstantaneous frequency, and further comprising an isolator unit forisolating signal segments whose instantaneous frequency approaches saidcharacteristic frequency as segments containing primary echoes.
 3. Theultrasonic transceiver apparatus of claim 2 further comprising a windowunit for windowing said received signal using a windowing length chosento provide windows with an expectation of a single primary echo.
 4. Theultrasonic receiving apparatus of claim 3, wherein said signal processoris further configured to find a point of appearance of a primary echo ina received signal by successively dividing said curve and fitting to alinear functions and calculating a point at which a corresponding errorfunction is minimized.
 5. The ultrasonic receiving apparatus of claim 4,further configured with a location unit to determine a distance to afirst feature wall from said point of appearance.
 6. The ultrasonicreceiving apparatus of claim 5, wherein said location unit is configuredto use a second point of appearance of a further primary echo todetermine a distance to a second feature wall, the signal processorfurther comprising a monitoring unit for monitoring a distance betweensaid first feature wall and said second feature wall as an indicator ofablation progress.
 7. The ultrasonic apparatus of claim 1, wherein saidsignal processor comprising a convolution unit for convolving anexcitation signal with the received signal to carry out said isolationof the primary echo.
 8. The ultrasonic apparatus of claim 1, whereinsaid signal processor comprises a Fourier component analyzer forisolating segments having a principle Fourier component whichcorresponds to a body-characteristic frequency.
 9. The ultrasonicapparatus of claim 1, wherein said signal processor comprises a coherentsummation unit for carrying out data summation such as to preserveamplitude and shift signals to a same phase.
 10. The ultrasonicapparatus of claim 9, wherein said coherent summation unit is configuredto perform coherent summation, said coherent summation comprisingbuilding an auxiliary matrix of phase weights, making a Hilberttransform and multiplying to bring the entire signal to the same phase,therewith to create an in-phase sum.
 11. The ultrasonic apparatus ofclaim 1, further comprising a reference subtracting unit configured tosubtract a reference from the transceiver signal by averaging severalsignal samples.
 12. An ultrasonic transceiver method for intracorporealuse, the method comprising: placing an undamped ultrasonic transceiverin a confined intracorporeal space, the transceiver having acharacteristic frequency exciting said transceiver at an instantaneousexcitation frequency to produce an ultrasonic ablation beam for ablatingsurrounding tissues using ablation pulses; at intervals between saidablation pulses providing monitoring excitation to elicit primary echosignals returning from surrounding tissues; isolating said primary echosignals from ringing, secondary echoes and extraneous noise alsoreceived from said transceiver using presence or absence of saidinstantaneous excitation frequency as an isolation criterion.
 13. Theultrasonic transceiver method of claim 12, wherein said isolationcomprises: obtaining an envelope of received signal minus excitationsignal from said undamped ultrasonic transceiver and using a globalphase and local slopes as an estimate of said instantaneous frequency,isolating those signal segments whose frequency approaches saidinstantaneous excitation frequency.
 14. The ultrasonic transceivermethod of claim 13 further comprising windowing said received signalusing a windowing length chosen to provide windows with an expectationof a single primary echo.
 15. The ultrasonic receiving method of claim14, further configured to find a point of appearance of a primary echoin a received signal by successively dividing said curve, fitting to alinear functions and calculating a point at which a corresponding errorfunction is minimized.
 16. The ultrasonic receiving method of claim 15,comprising determining a distance to a first feature wall from saidpoint of appearance.
 17. The ultrasonic method of claim 16, comprisingusing a second point of appearance of a further primary echo todetermine a distance to a second feature wall, and monitoring a distancebetween said first feature wall and said second feature wall as anindicator of ablation progress.
 18. The ultrasonic method of claim 12,comprising convolving an excitation signal with the received signal tocarry out said isolation of the primary echo.
 19. The ultrasonic methodof claim 12, comprising isolating segments having a principle Fouriercomponent which corresponds to a body characteristic frequency.
 20. Theultrasonic method of claim 12, comprising carrying out coherent datasummation such as to preserve amplitude and shift signals to a singlephase.
 21. The ultrasonic method of claim 20, wherein said coherentsummation comprises: making an auxiliary weights matrix evaluation;carrying out a Hilbert transformation; multiplication to bring allsignals to the same phase; and performing an in-phase summation.
 22. Anultrasonic transceiver apparatus for intracorporeal use, the apparatuscomprising: an undamped ultrasonic transceiver for placing in a confinedintracorporeal space, the transceiver having a characteristic frequencyand for receiving excitation at said characteristic frequency to producean ultrasonic ablation beam for ablating surrounding tissues and beingfurther for receiving excitation by primary echo signals returning fromsurrounding tissues; a signal processor connected to said transceiverconfigured to isolate said primary echo signals from ringing, secondaryechoes and extraneous noise also received from said transceiver, saidsignal processor using correlation with a body-characteristic frequencyas an isolation criterion.
 23. The apparatus of claim 22, wherein saidbody characteristic frequency is one member of the group consisting ofpulse and breathing rate.
 24. The apparatus of claim 22, wherein saidsignal processor is configured to obtain a power spectrum of a signalextracted from said transceiver and to identify said primary echoes frompeaks in said power spectrum at said body-characteristic frequency. 25.The apparatus of claim 22, further comprising a coherent summation unit.26. The apparatus of claim 22, further comprising a convolution unit.27. A method of providing controlled thermal damage to a tissue,comprising: identifying locations of boundary walls of said tissue;applying energy to said tissue; during said applying, monitoring changesin locations of said boundary walls as indicators of an effect of saidapplying said energy on said tissue; and controlling said thermal energyaccording to said effect.
 28. The method of claim 27, wherein saidmonitoring and said applying are carried out from within a blood vessel.29. The method of claim 27, wherein said monitoring and said applyingare carried out using ultrasonics.
 30. The method of claim 27, whereinsaid effect on said tissue is thermal shrinkage.
 31. The apparatus ofclaim 29, wherein said applying comprises applying a non-focusedultrasonic ablation beam.
 32. The apparatus of claim 1, wherein saidultrasonic ablation beam is not focused.
 33. The method of claim 12,wherein said ultrasonic ablation beam is not focused.